Quality standards in the specialty coffee industry established during the 1950's and 1960's state that coffee extraction of approximately 20% (usually a range of ˜18-22%) will achieve the best quality brewed coffee, using various brew methods. Over the ensuing years these established standards have been generally referred to as the gold-cup standard, and have been accepted internationally by many institutions, educational establishments, and standards committees. The precise extraction (solubles yield) and strength (solubles concentration) may be varied for particular coffees, to achieve finely tuned recipes for particular coffee cultivars, climates and growing regions, also known as terroir, and other characteristics.
Extraction (solubles yield) refers to the percentage of the dry coffee by weight that is removed by dissolving in water during the brewing process. Up to 30% of the available soluble solids in ground coffee can be extracted, with most of the remaining 70% being cellulose, and not soluble in water. However, generally speaking, extracting more than 22% will begin to sharply increase those components in coffee that contribute to bitter taste defects associated with over-extraction. Extracting less than 18% is generally associated with weak, under-developed taste defects often associated with sour taste defects, especially at higher concentrations, such as with espresso.
The finished brew is a balance between extraction (solubles yield) served at the desired strength (solubles concentration). Over-extracting, which results in taste defects known as bitter, can be known as strong-bitter at higher concentrations, or weak bitter at lower concentrations. Similarly, under-extracting, which results in taste defects known as sour or under-developed, can be known as strong-under-developed at higher concentrations, or weak under-developed at lower concentrations. The highest quality coffee will have rich aroma, fullness of body, delicacy of flavor as well as clarity and unique character—all optimally balanced. Achieving these attributes requires a significant degree of precision during the coffee brewing process.
Strength (solubles concentration) refers to the measured amount of solids extracted into the final coffee solution. Strength may be expressed as the percentage total dissolved solids (% TDS). For example, for 100 grams of a coffee measuring 8.5% TDS, there are 91.5 grams of water, and 8.5 grams of dissolved coffee solids in solution.
To brew coffee, such as espresso, a specific dose of selected ground coffee is uniformly distributed and tamped into an espresso basket filter. Water heated to a predetermined temperature and pressure is injected through the coffee basket filter while the shot of espresso coffee is brewed into a cup or other vessel. In order to obtain maximum sweetness, aroma, body and flavor it is desirable for the barista or other user to be able to consistently pull shot after shot of high quality coffee with uniform concentration and extraction yield.